Continuous separation processes are commonly used for the selective adsorption of para-xylene from a mixture of C8 aromatics. Generally, the processes use a solid adsorbent that preferably retains the para-xylene in order to separate the para-xylene from the rest of the mixture. Often, the solid adsorbent is in the form of a simulated moving bed, where the bed of solid adsorbent is held stationary, and the locations at which the various streams enter and leave the bed are periodically moved. The adsorbent bed itself is usually a succession of fixed sub-beds or modules. The shift in the locations of the liquid input and output in the direction of the fluid flow through the bed simulates the movement of the solid adsorbent in the opposite direction. Moving the locations of the liquid input and output is accomplished by a fluid tracking device known generally as a rotary valve which works in conjunction with distributors located between the adsorbent sub-beds. The rotary valve accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific distributors located between the adsorbent sub-beds. After a specified time period, called the step time or hold period, the rotary valve advances one index to the next valve position and redirects the liquid inputs and outputs to the distributors immediately adjacent and downstream of the previously used distributors. Each advancement of the rotary valve to the next valve position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. In one commercial process, the step time is uniform for each of the valve steps in a valve cycle, and is generally about 60 seconds or so. A typical process contains 24 adsorbent sub-beds, 24 distributors located between the 24 adsorbent sub beds, two liquid input lines, two liquid output lines, and associated flush lines.
The principle liquid inputs and outputs of the adsorbent system consists of four streams, which are the feed, the extract, the raffinate, and the desorbent. Each stream flows into or out of the adsorbent system at a particular flow rate, and each rate is independently controlled. The feed, which is introduced to the adsorbent system, contains the para-xylene that is to be separated from the other components in the feed stream. The desorbent, which is introduced to the adsorbent system, contains a liquid capable of displacing feed components from the adsorbent. The extract, which is withdrawn from the adsorbent system, contains the separated para-xylene, which was selectively adsorbed by the adsorbent, and the desorbent liquid. The raffinate, which is withdrawn from the adsorbent system, contains other C8 aromatic components of the feed that are less selectively adsorbed by the adsorbent, and desorbent liquid. There also may be associated flush streams introduced to and withdrawn from the adsorbent system. The four principal streams are spaced strategically throughout the adsorbent system and divide the sub-beds into four zones, each of which performs a different function.
Zone I contains the adsorbent sub-beds located between the feed input and the raffinate output, and the selective adsorption of the para-xylene takes place in this zone. Zone II contains the adsorbent sub-beds located between the extract output and the feed input, and the desorption of components other than the para-xylene takes place in this zone. Zone III contains the adsorbent sub-beds located between the desorbent input and the extract output, and the para-xylene is desorbed in this zone. Finally, Zone IV contains the adsorbent sub-beds located between the raffinate output and the desorbent input. The purpose of zone IV is to prevent the contamination of the para-xylene with other components.
A common practice in the industry is to determine the compositional profile of the para-xylene simulated moving bed separation process either by on-line gas chromatography analysis, or by off-line laboratory analysis. The on-line gas chromatography analysis typically requires about 10 minutes per analysis, which is considerably greater than the usual step time of the rotary valve. Therefore, only selected valve positions can be sampled and analyzed. Generally, only Zone II near the extract output and Zone IV near the desorbent input are sampled and analyzed. The data provided by this on-line gas chromatography procedure is useful for detecting some process upsets, but unfortunately analyzing the composition of only two valve positions provides limited information regarding the performance of the separation process and is only minimally useful for precise separation process control.
A more thorough determination of the compositional profile of the para-xylene simulated moving bed separation process is accomplished using off-line laboratory gas chromatography analysis to determine the values of the concentrations of the components in the samples for each valve position in a valve cycle. The measured concentrations are then plotted versus their relative valve positions to form what is generally called a pump-around profile. Using the pump-around profile, the recovery purity of the para-xylene can be calculated and the degree of optimization of the separation may be assessed. From this, for example, needed changes in the step time and/or liquid stream flow rates may be determined and implemented. The drawbacks to assessing the separation process in this fashion are the time delay between sampling and delivery of the analytical results, where the latter are used to determine whether or what process changes should be made; the labor involved to manually collect the stream samples; and the personal exposure of the operator manually collecting the stream samples from the process. Since the analysis is performed off-line, the time delay may be from one to several days long and can lead to plant disruption. Because of these drawbacks, refiners generally only perform this procedure about once every six months or if there is a problem with the separation process.
Accordingly, it is desirable to provide systems for the separation of para-xylene from other hydrocarbon components and processes for determining the pump-around profile of these systems to provide rapid and frequent compositional profiles with low system maintenance, requiring minimal operator time and labor, and without plant disruption. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention in the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.